This invention relates to pattern inspection for detection of defects, such as a short circuit and the breaking of wire, and foreign matter. Specifically, the invention relates to a pattern inspection apparatus and method for use in the manufacture of a semiconductor wafer, a liquid crystal display, and a photomask. Note that in the following description, the term “defect” includes foreign matter, and “a pattern to be inspected” will be referred to as an “inspection pattern” hereinafter.
As far as the present inventors are aware, regarding pattern inspection technology for detecting a defect in an inspection pattern, the following technologies have been developed.
JP-A No. 318326/1995 discloses an inspection apparatus in which an image of an inspection pattern is detected by an imaging element, such as an image sensor, while the inspection pattern is being moved, and a detected image signal and an image signal that represents the detected image signal delayed by a predetermined time are compared in terms of gray-scale value, whereby a nonconformity is recognized as a defect.
Further, JP-A No. 320294/1996 discloses a technique for defect inspection of an inspection pattern. According to this technique, in inspection patterns of a semiconductor wafer in which a region of high pattern density, such as a memory mat part, and a region of low pattern density, such as a peripheral circuit, are mixed in a single chip, a digital image signal obtained by A/D converting the detected image signal is subjected to gray-scale translation. This gray-scale translated image signal is then compared to an image signal that was subjected to gray-scale translation and is to be compared while the two are aligned so that the brightness or contrast between the high density region and the low density region of the inspection pattern becomes a predetermined relationship according to a frequency distribution of brightness on a detected image, and thereby a minute defect can be detected with high accuracy.
Moreover, a method of inspecting the pattern of a photomask is disclosed by JP-A No. 78668/1998. JP-A No. 78668/1998 discloses a technique in which a UV laser beam, such as an excimer laser beam, is used as a light source, the mask is illuminated uniformly with the UV light whose coherence was reduced by rotating a diffuser plate inserted into an optical path, features are calculated from obtained image data of the mask, and the quality of the photomask is judged. Moreover, a projection aligner using an excimer laser is disclosed in JP-A No. 226317/1984, JP-A No. 231924/1987, etc.
On the other hand, the present inventors have examined the various techniques used in pattern inspection technology for detecting a defect in an inspection pattern, as mentioned above, and have come to the following conclusions.
For example, in LSI manufacture in recent years, a circuit pattern formed on a wafer has become more minute, having been reduced down to 200 nm or less in pattern width, in response to the need for a high degree of integration, which has come up to the resolution limit of the optical system for performing pattern inspection. For this reason, enhancement of the NA of the objective lens for inspection to higher values and the application of super resolution technology are being advanced.
However, enhancement of the NA of the objective lens for inspection to higher values has reached a physical limit. Therefore, making the wavelength used for the inspection shorter toward the region of ultraviolet light (UV light) and far ultraviolet light (DUV light) is an essential approach.
In the inspection, since it is necessary to perform inspection at high speed, a method of scanning a narrow-focused laser beam on the sample cannot be used. Conversely, if the laser beam is expanded to the whole visual field for illumination, speckle occurs and overshoot/undershoot called ringing occurs at the edge part of a circuit pattern; therefore, an excellent quality of image production cannot be obtained.
On the other hand, the inspection device has been changing, and the structure of the inspection pattern has become complex and diversified, such as memory products that are formed with repeated patterns and logic products that are formed with non-repeated patterns. Therefore, it has become difficult to find a targeted defect surely.